fIRE & eXPLOSION

1. What is a fire?
A fire is a complex chain reaction
where a fuel combines with oxygen
to generate heat, smoke, and light.
Most chemicals fires will be
triggered by one of the following
ignition sources: sparks, static
electricity,
heat, or flames from another fire.
Additionally,

2. if a chemical is above its auto
ignition temperature it will
Spontaneously catch on fire without
an external ignition source.
There are several properties that
measure how readily—that is, how
easily—a chemical will catch on
fire.

3. Here we'll discuss three of these
properties: volatility, flash point,
and flammability limits. Volatility is
a measure of how easily a chemical
evaporates. A flammable liquid
must begin to evaporate—forming
a vapor above the liquid—before it
can burn.

4. The more volatile a chemical, the
faster it evaporates and the quicker
a flammable vapor cloud is formed.
The flash point is the lowest
temperature where a flammable
liquid will evaporate enough to
catch on fire if an ignition source is
present. The lower the flash point,
the easier it is for a fire to start.
Flammability limits,

5. called the Lower Explosive Limit
(LEL) and the Upper
Explosive Limit (UEL), are the
boundaries of the flammable region
of a vapor cloud. These limits are
percentages that represent the
concentration of the fuel—that is,
the chemical—vapor in the air. If
the chemical vapor comes into
contact with an ignition source,

6. it will burn only if its fuel-air
concentration is between the LEL
and the UEL. To some extent,
these properties are interrelated—
chemicals that are highly volatile
and have a low flash point will
usually also have a low LEL.
Once the chemical catches on fire,
three things need to be present to
keep the fire going:

7. fuel (the chemical), oxygen, and
heat. This is often referred to as the
fuel triangle. If any one of those
components is eliminated, then the
fire will stop burning.
Like other reactions, a fire can also
generate byproducts—smoke, soot,
ash, and new chemicals formed in
the reaction.
Some of these reaction byproducts
can be hazardous themselves.

8. While ALOHA cannot model all the
complex processes that happen in
a fire (like the generation and
distribution of byproducts), it can
predict the area where the heat
radiated by the fire—called thermal
radiation—could be harmful.
Thermal radiation is the primary
hazard associated with fires.

9. However, it is also important to
consider the hazards associated
with any secondary fires and
explosions that may occur.
Many of the chemicals in ALOHA's
chemical library are flammable, in
addition to being toxic and volatile
enough to be potential air hazards.
For those chemicals, you can
model not only the toxic threat
posed by the release of that

10. chemical, but also the fires and/or
explosions that the chemical could
potentially cause. However,
ALOHA cannot model these threats
at the same time.
If a flammable and toxic chemical—
such as acrolein—has been
released, run a toxic gas dispersion
scenario first. Next, run all of the
appropriate fire and explosions
scenarios.

11. Finally, consider all of the threat
zone plots (the estimates will vary
with each scenario) and any
additional site-specific data and use
that information to decide how you
are going to respond to the
incident. In many situations
involving a flammable and toxic
chemical,

12. the area encompassed by the toxic
threat zone will be greater than the
threat zones associated with fire
and explosion scenarios. It is
essential that you evaluate all of
the scenario options before
developing your response plan.

13. What is an explosion?
The most basic definition of an
explosion is a sudden, intense
release of energy that often
produces a loud noise, high
temperatures, and flying debris,
and generates a pressure wave.
There are many types of explosions
and the causes and effects will
vary.

14. ALOHA primarily models
explosions that are the result of
accidents involving industrial
chemicals. Intentional explosions
will generally—but not always—
result in greater hazard damage.
Consider three primary hazards
when dealing with an explosion:
thermal radiation, overpressure,
and

15. hazardous fragments (flying
debris). All three of these hazards
are not present in every explosion
and the severity of the hazard will
depend on the explosion. These
hazards typically last only for a brief
period directly following the
explosion. However, it is important
to consider the potential for
secondary explosions and fires to

16. occur before deciding that these
hazards no longer exist.
Overpressure. A major hazard
associated with any explosion is
overpressure. Overpressure, also
called
a blast wave, refers to the sudden
onset of a pressure wave after an
explosion. This pressure wave is
caused

17. by the energy released in the initial
explosion—the bigger the initial
explosion, the more damaging the
pressure wave. Pressure waves
are nearly instantaneous, traveling
at the speed of sound.
Although a pressure wave may
sound less dangerous than a fire or
hazardous fragments, it can be just
as

18. damaging and just as deadly. The
pressure wave radiates outward
like a giant burst of air, crashing
into
anything in its path (generating
hazardous fragments). If the
pressure wave has enough power
behind it, it
can lift people off the ground and
throw them up against nearby

19. buildings or trees. Additionally,
blast
waves can damage buildings or
even knock them flat—often
injuring or killing the people inside
them. The
sudden change in pressure can
also affect pressure-sensitive
organs like the ears and lungs. The
damaging

20. effects of the overpressure will be
greatest near the source of the
explosion and lessen as you move
farther
from the source.
When you use ALOHA to predict an
explosion's effects, assess the
surroundings at the explosion site
as
you interpret ALOHA's threat zone
plot. Large objects (like trees and

21. buildings) in the path of the
pressure
wave can affect its strength and
direction of travel. For example, if
many buildings surround the
explosion
site, expect the actual overpressure
threat zone to be somewhat smaller
than ALOHA predicts. But at the
same time, more hazardous
fragments could be generated as

22. the blast causes structural damage
to those
buildings.
Jet fires
A jet fire, also referred to as a flame
jet, occurs when a flammable
chemical is rapidly released from
an opening in a container and
immediately catches on fire—much

23. like the flame from a blowtorch.
ALOHA
can model a jet fire from the Gas
Pipeline and Tank sources. For the
Tank source, ALOHA can model
gas
and two-phase jet fires. A two-
phase jet fire occurs when a gas
that has been liquefied under
pressure is released. Because the
liquid evaporates as it escapes, the

24. chemical is released as an aerosol
spray—that is,a mixture of gas and
tiny liquid droplets.
ALOHA assumes the jet fire
release is oriented vertically,
although the wind can tilt the
flames in the
downwind direction.
Thermal radiation is the primary
hazard associated with a jet fire.

25. Other potential jet fire hazards
include
smoke, toxic byproducts from the
fire, and secondary fires and
explosions in the surrounding area,
although ALOHA does not model
these hazards.
In some cases, heat from the jet
fire may weaken the tank and
cause it to fail completely—in which
case, a

26. BLEVE may occur. Typically, a
BLEVE poses a greater threat than
a jet fire. If the chemical inside the
tank is likely to BLEVE (for
example, if the tank contains a
liquefied gas), in addition to
modeling the
scenario as a jet fire, you should
also rerun the scenario as a BLEVE
to compare the size of the threat
zones.

27. Pool fire
A pool fire occurs when a
flammable liquid forms a puddle on
the ground and catches on fire.
ALOHA
only models pool fires on land; it
does not model pool fires on water.
Thermal radiation is the primary
hazard associated with a pool fire.
Other potential pool fire hazards

28. include smoke, toxic byproducts
from
the fire, and secondary fires and
explosions in the surrounding area
(although ALOHA does not model
these hazards).
In some cases, heat from the pool
fire may weaken a leaking tank and
cause it to fail completely—in which
case, a BLEVE may occur.
Typically, a BLEVE poses a greater

29. threat than a pool fire. If the
chemical
inside the tank is likely to BLEVE
(for example, if the tank contains a
liquefied gas), you may want to
model the situation first as a pool
fire and then rerun the scenario as
a BLEVE to compare the size of the
threat zones.

30. BLEVEs
BLEVE stands for Boiling Liquid
Expanding Vapor Explosion.
BLEVEs typically occur in closed
storage tanks that contain a
liquefied gas, usually a gas that has
been liquefied under pressure. A
gas can be liquefied by either
cooling (refrigerating) it to a
temperature below its boiling point
or by storing it at a high pressure.

31. Although both flammable and
nonflammable liquefied gases may
be involved in a BLEVE,
ALOHA only models flammable
liquid BLEVEs.
Propane is an example of a
chemical that has been involved in
many BLEVE accidents. Most
propane tanks at service stations
contain liquid propane. These tanks
are neither insulated nor

32. refrigerated, so the tank contents
are at ambient temperature. Since
the ambient temperature is almost
always significantly above
propane's boiling point of -43.7 ºF,
the tanks are highly pressurized.
A common BLEVE scenario
happens when a container of
liquefied gas is heated by fire,
increasing the pressure within the
container until the tank ruptures

33. and fails. When the container fails,
the chemical is released in an
explosion. If the chemical is above
its boiling point when the container
fails, some or all of the liquid will
flash-boil—that is, instantaneously
become a gas. If the chemical is
flammable, a burning gas cloud
called a fireball may occur if a
significant amount of the chemical
flash-boils. ALOHA assumes

34. that any liquid not consumed in the
fireball will form a pool fire.
ALOHA estimates the thermal
radiation hazard from a fireball
and/or a pool fire. Other potential
BLEVE
hazards include overpressure,
hazardous fragments, smoke, and
toxic byproducts from the fire
(although

35. ALOHA does not model these
hazards). ALOHA focuses on the
thermal radiation because in most
BLEVEs thermal radiation impacts
a greater area than the
overpressure and is the more
significant threat.
Fireball. When you model a
BLEVE, ALOHA assumes that a
fireball will form. The fireball is

36. made up of both the chemical that
flash-boils when the tank fails and
the chemical that sprays out as an
aerosol during the explosion.
ALOHA estimates that the amount
of chemical in the fireball is three
times the
amount of chemical that flash boils.
Any liquid that does not participate
in the fireball will form a pool fire.

37. When you choose to model a
BLEVE situation in ALOHA, the
program estimates the thermal
radiation
from both fires; it is not necessary
to run an additional Pool Fire
scenario. The primary hazard
associated
with a fireball is thermal radiation.
However, if there are other

38. chemicals near the fireball, it can
trigger
additional fires and explosions.
Explosion and hazardous
fragments. In a BLEVE, a high-
pressure explosion typically causes
the
container to fragment. As the
container breaks apart, it may strike
objects in the surrounding area and

39. create additional debris. The
container fragments and other
debris—hazardous fragments—are
swept up in
the explosion and rapidly propelled
by the explosion over a wide area.
ALOHA does not model the
dispersion of hazardous fragments
or overpressure (blast force) in a
BLEVE. If a BLEVE is likely to
occur,

40. first responders must take the
necessary precautions to protect
themselves and others from the
overpressure
and hazardous fragments.
Flash fires (flammable area)
When a flammable vapor cloud
encounters an ignition source, the
cloud can catch fire and burn
rapidly in

41. what is called a flash fire. Potential
hazards associated with a flash fire
include thermal radiation, smoke,
and toxic byproducts from the fire.
ALOHA will predict the flammable
area of the vapor cloud—that is,
the area where a flash fire could
occur at some time after the
release. The flammable area is
bounded by the

42. Lower Explosive Limit (LEL) and
the Upper Explosive Limit (UEL).
These limits are percentages that
represent the concentration of the
fuel—that is, the chemical—vapor
in the air. If the chemical vapor
comes into contact with an ignition
source, it will burn only if its fuel-air
concentration is between the LEL

43. and the UEL, because that portion
of the cloud is already pre-mixed to
the right mixture of fuel and air for
burning to occur.
If the fuel-air concentration is below
the LEL, there is not enough fuel in
the air to sustain a fire or an
explosion—it is too lean. If the fuel-
air concentration is above the UEL,
there is not enough oxygen to

44. sustain a fire or an explosion
because there is too much fuel—it
is too rich. (This is similar to an
engine
that cannot start because it has
been flooded with gasoline.) If a
flash fire occurs, the part of the
cloud
where the fuel-air concentration is
above the UEL may continue to

45. slowly burn as air mixes with the
cloud.
You might expect that the LEL
could be used as the LOC to
determine the areas in which a fire
might
occur. However, the concentration
levels estimated by ALOHA are
time-averaged concentrations. In
an

46. actual vapor cloud, there will be
areas where the concentration is
higher than the average and areas
where
the concentration is lower than the
average. This is called
concentration patchiness. Because
of
concentration patchiness, there will
be areas, called pockets, where the
chemical is in the flammable range

47. even though the average
concentration has fallen below the
LEL. (ALOHA uses a shorter
averaging time
when estimating the flammable
areas, to help compensate for this
effect, but it cannot completely
compensate for this effect.) Some
experiments have shown that flame
pockets can occur in places where

48. the average concentration is above
60% of the LEL. ALOHA uses 60%
of the LEL as the default LOC for
the red threat zone. Another
common threat level used by
responders is 10% of the LEL.
ALOHA uses this
concentration as the default LOC
for the yellow threat zone.
Vapor cloud explosions

49. When a flammable chemical is
released into the atmosphere, it
forms a vapor cloud that will
disperse as it
travels downwind. If the cloud
encounters an ignition source, the
parts of the cloud where the
concentration is within the
flammable range (between the LEL
and UEL) will burn. The speed at
which the

50. flame front moves through the
cloud determines whether it is a
deflagration or a detonation (see
next
page). In some situations, the cloud
will burn so fast that it creates an
explosive force (blast wave). The
severity of a vapor cloud explosion
depends on the chemical, the cloud
size at the time of ignition, the type

51. of ignition, and the congestion level
inside the cloud. The primary
hazards are overpressure and
hazardous
fragments. ALOHA can help you
model the overpressure hazard.
No, there is no difference. The two
terms can be used interchangeably.
Some people

52. may prefer to use the terms Lower
Flammable Limit (LFL) and Upper
Flammable
Limit (UFL), particularly if they are
only concerned with fires.
Deflagration and detonation. The
destructive blast force of a vapor
cloud explosion depends in part
on how quickly the explosion
spreads—that is, the rate at which

53. its flame front travels. Once an
explosion
has been triggered, a flame front
will spread through the flammable
vapor cloud, igniting areas where
the
concentration is in the flammable
range. The explosion produces a
pressure wave that spreads out into
the

54. surrounding area, causing damage
to people and property. The greater
the speed of the flame front, the
more intense the pressure wave
(overpressure), and the greater the
destructive force of the explosion.
For most accidental explosions, the
flame front will travel relatively
slowly in what is called a
deflagration. For example, a typical
deflagration flame front (for

55. hydrocarbon combustions) travels
about
2.2 miles per hour, or 1 meter per
second (Lees 2001). For intentional
explosions (and worst-case
accidental explosions), the flame
front travels rapidly in what is called
a detonation. For example, a typical
detonation flame front (for
hydrocarbon combustions) travels

56. about 5,600 miles per hour, or
2,500 meters
per second (Lees 2001). In some
situations, a deflagration flame front
can accelerate into a detonation
flame front. Accidental explosions
that result in a high-speed
deflagration or a detonation are
more likely
to occur in areas of high congestion
and confinement.

57. Congestion and confinement.
Congestion is a concept used to
quantify the way small structures
within the vapor cloud affect the
severity of the explosion.
Congestion refers to the density of
obstacles
that generate turbulence. Obstacles
of this nature are generally small,
like a shrub, and do not impede the

58. flame front. Larger objects, like a
building, can impede the flame
front, so they should not be
considered
obstacles for the purposes of
congestion. Greater turbulence
allows the flame front to accelerate,
thereby
generating a more powerful blast
wave (i.e., greater overpressure).

59. ALOHA uses two congestion levels:
congested and uncongested.
ALOHA's blast estimates are based
on
experiments that used a volume
blockage ratio (volume occupied by
obstacles within the cloud divided
by
cloud volume) of less than 1.5% for
an uncongested cloud and greater
than 1.5% for a congested cloud.

60. Estimating the level of congestion
in a non-laboratory setting is
difficult, but the following examples
might
be helpful. Uncongested zones
include: parking lots, open fields,
suburban neighborhoods, and most
urban
environments. Generally, a
congested zone has so many

61. closely spaced obstacles that it is
difficult or
impossible to walk through it. It is
uncommon for this level of
congestion to be found throughout
the entire
vapor cloud. However, pipe racks in
industrial facilities and some
forested areas (where the trees and

62. branches are closely spaced) may
be characterized as congested
areas.